The present invention relates to a focus monitoring method which is suited for controlling a focus condition of exposure equipment used for photolithography in manufacturing a semiconductor device or a liquid crystal display device.
In the photolithography process of manufacturing a semiconductor device or a liquid crystal display device, the focus can be dislocated or deviated from the predetermined focus point because of the unevenness of the surface of a wafer caused by mechanical impreciseness of exposure equipment and/or irregularity of process condition. Such a focus-deviation can produce deviation of a pattern transferred onto the wafer and reduce the production yield.
According to a conventional method, a focus-monitoring pattern is provided in a reticule, which is transferred onto the wafer, and the dimension of the transferred focus-monitoring pattern is periodically monitored to detect the focus-deviation, then, the focus is corrected according to the result of the detection to prevent the reduction of the manufacturing yield.
With the recent advance of the technology for a very fine device pattern, it has been difficult to secure a sufficient process margin of the value of exposure and the depth of focus during the photolithography. Therefore, there is a demand for a new technology enabling to precisely monitor the focus of exposure equipment by utilizing a small process margin in order to prevent the reduction of the manufacturing yield.
Conventional focus-monitoring methods are described below. In non-patent document entitled “Edge Die Focus-Expose Monitoring and Comparison to Improve CD Distributions” (Proceedings of SPIE Vol. 5375), a reticule having a focus-monitoring pattern (mark) formed by a plurality of fine lines 15 such as shown in FIG. 12 has been used. The quantity or magnitude of retreat of the ends of such fine lines is sensitive to the focus. Accordingly, precise focus-monitoring is possible by monitoring the quantity of the end-retreat of the fine lines. In FIG. 12, a dimension L between the fine lines 15 of a focus-monitoring pattern is measured to substantially monitor the quantity of the end-retreat of the fine lines.
FIG. 13 shows the relationship between the dimension L and the focus position, wherein the focus position of the most suitable or optimum focus is located at the origin of the coordinate axes. The horizontal axis (x-axis) denotes the focus position and the vertical axis (y-axis) denotes the dimension L. In this specification, the optimum focus means the focus position at which the desired characteristics of a manufactured product are obtained. As shown in FIG. 13, the dimension L is variable substantially symmetrically in respect to the focus position. Data, which is shown in the FIG. 13, is obtained in advance before focus-monitoring in order to utilize the above-mentioned relationship between the focus position and the dimension L as a calibration curve. The dimension L at the shot of the optimum focus is measured and the actual focus position is determined from the dimension L of the optimum focus and the calibration curve. The difference between the obtained focus position and the optimum focus is the quantity of the focus-deviation.
This conventional method, however, has the following disadvantages.
As described above, the dimension L of a focus-monitoring pattern is changed substantially symmetrically with respect to the focus position (FIG. 13), which means that there is a plurality of solutions of the focus position even if the dimension L is determined. Therefore, the focus position is not determined uniquely from the calibration curve of the focus position and also the direction of the focus-deviation can not be determined. Accordingly, when the focus position is corrected, the correction in the direction of focus-deviation has been made in accordance with some assumption. For the above reasons, it has been difficult to set suitable focus conditions.
Also, the change of the dimension L with respect to the focus position is not sensitive in the vicinity of the focus position where the calibration curve passes the extreme value. It reduces the precision of the focus-monitoring. The optimum focus is usually set in the vicinity of the focus position at the extreme value of the calibration curve because the focus characteristics of the device region, which becomes a judging criterion for the optimum focus, are similar to those of the focus-monitoring pattern. For these reasons, it is natural that the precision of the focus-monitoring is reduced in the vicinity of the optimum focus.
In Japanese Patent Application Kokai Number 2001-189264, it is proposed that the defocus characteristics of two focus-monitoring patterns are changed by intentionally providing a phase difference to lights passing through the two focus-monitoring patterns in order to precisely monitor the focus-deviation including the direction.
FIG. 14 illustrates the outline of such a method. FIG. 14(a) is a sectional view of a reticule. The reticule is composed of three layers; a glass board 161, a semi-transparent film 162, and a light shielding film 163. FIG. 14(b) is a plan view of the reticule, FIG. 14(c) is a plan view of a focus-monitoring pattern 140, and FIG. 14(d) is a partly enlarged plan view of the focus-monitoring pattern 140. The focus-monitoring pattern 140 has a semi-transparent film section 801 in the periphery, a shielding film section 802 in the center, first monitor marks 101 constituting opening sections, and second monitor marks 102 constituting opening sections. Each of the first and second monitor marks 101 and 102 has a rectangular pattern and a fine tapered pattern (FIG. 14(d)) formed on one side of the rectangular pattern. For the purpose of providing a phase difference of right (90 degrees) angles between the exposure light passing through the semi-transparent film and the exposure light passing through the opening section, a portion of the glass board 161, where the first monitor mark 101, is located is etched down to a depth of, for example, 124 nm, as shown in FIG. 14(a).
FIG. 15 shows a difference in the quantity or magnitude of the focus deviation relative to the defocus of the first and second monitoring marks 101 and 102. FIG. 16 shows the magnitude of deviation S (=S2−S1) of the relative positions between the first and second monitoring marks 101 and 102. FIG. 17 shows the relationship between the defocus and the magnitude of deviation S from the optimum focus. Since the magnitude of deviation S is monotonically increased, the magnitude of deviation S can be monitored including the direction of deviation S.
As described above, in Japanese Patent Application Kokai Number 2001-189264, the special technique is required so that a part of the glass reticule is not useful for the monitoring.
In addition, as for one of the two monitoring marks, as shown in FIG. 15, since the measurement is taken in the vicinity of the focus position at the extreme value of the calibration curve, the precision of the focus-monitoring is reduced in the same way as the above-mentioned non-patent document.